In recent years, from an increase in awareness and a social demand of safety and ease of mind, and the like, a degree of attention to an infrared sensor detecting infrared rays emitted by a living body is increasing. The infrared rays are classified, depending on a wavelength band thereof and uses, into near-infrared rays (for example wavelengths of 0.7 μm to 2 μm), mid-infrared rays (for example, wavelengths of 3 μm to 5 μm), and far-infrared rays (for example, wavelengths of 8 μm to 13 μm), and there can be cited, as the uses, a touch sensor and a near-infrared camera in the near-infrared ray, a gas analysis and a mid-infrared spectroscopic analysis (functional group analysis) in the mid-infrared ray, a nightvision, a thermoviewer, and the like in the far-infrared ray.
For the infrared sensor, depending on the uses, optical devices such as an optical filter, an optical window, a lens to condense infrared rays, and a polarizing device for removing reflected light are provided in front thereof. As materials for such optical devices, conventionally, infrared transmitting materials such as germanium (Ge), silicon, zinc sulfide (ZnS), selenium sulfide, and a halide are being used.
However, because Ge, silicon, ZnS, and selenium sulfide are crystals, they are poor in processability and difficult to process into a complex shape such as an aspherical lens. Therefore, there have been problems of difficulty in mass production and further difficulty in miniaturization of the infrared sensor. In addition, the halide is poor in environmental resistance and undergoes a remarkable reduction in a transmitting property due to moisture, and therefore it has been unsuitable for the nightvision for external use such as on-vehicle and monitoring uses.
Thus, as an infrared transmitting material which has no problem of the environmental resistance, and is processed relatively easily and also allows aspherical lens processing by press molding, chalcogenide glass containing a chalcogen element as a main component is proposed (for example, refer to Patent References 1 to 4 (JP-A No. H06-191861, JP-A No. H05-85769, JP-A No. 2009-161374 and JP-A No. 2015-129072).).